A rolling stone gathers no moss……but your roof sure does, in addition to any number of other growing things.  Organic growth on a prepainted metal roof is an aesthetic problem that may become a more serious problem if not addressed. This blog offers a simple approach to dealing with the problem.

A roof can look like this…

…or this

These biological growths thrive on moisture and organic debris. The more water and the more debris, the greater the chance of growth. Another critical factor is the amount of shade your roof receives. A well-shaded roof prevents the evaporation of moisture, creating excellent growing conditions.

But what is this stuff? It is usually some combination of moss, algae, and lichen. Sometimes there is dark buildup that may look like dirt, but it is more likely algae growth called gloeocapsa magma, also known as blue-green algae. A similar material in the same family is cyanobacteria. The “cyano” in this bacteria refers to its blue-green color. This growth can turn very dark in color over time. Algae is an airborne plant, so it spreads relatively easily. It thrives in humid conditions because it needs heat, moisture, and nutrients to grow. 

Lichen is a combination of algae and fungi. Lichen is not technically a plant at all, but rather a type of fungus that exists in a symbiotic relationship with algae. There are four types of lichen with varying appearances, and it is easy to mistake lichen for moss. You’ve probably seen lichen growing on the north side of trees, where large green patches are readily seen. 

And moss…well, it looks like moss. We’ve all seen it on a forest floor.

Prepainted metal is not a food source for this organic growth; rather, airborne organic material that falls onto the roof, such as leaves and bird droppings, is the source of food for the organic growth. 

Although it may sound confusing to know what is growing on your roof, the solution to removing the growth is the same: bleach! Not the stuff right out of that white plastic jug, but a diluted version, which also contains some mild detergent to wet the surface of the prepainted metal roof. 

Here’s a formulation to consider:

1. 1/3 cup of powdered laundry detergent (such as Tide, All, Arm & Hammer, Gain, etc.)
2. 1 quart of bleach (5% solution)
3. 3 quarts of water

Grab a plastic pail, make the above solution, and you’re almost ready. But first, there are several precautions to take, as follows:

1. Test the solution you are using before wetting the entire roof surface. Pick a spot on the roof that is covered or less noticeable to test the cleaning solution. That way, you will know if the cleanser works and will ensure it doesn’t cause damage to the system.
2. Spray this solution onto the affected area, or use a soft brush or rag and lightly scrub the surface until the area is free of organic growth.
3. Always wear protective goggles and gloves when handling the above solution. 
4. Do not deviate from the above solution. Do not increase the amount of soap and bleach.
5. Never use abrasive materials, steel wool, sandpaper, scouring powders, or a wire brush to clean the roof.
6. Never use a high-pressure washer to clean your roof.
7. Never use paint thinners, solvents, or paint removers to clean a prepainted metal roof.
8. Ladder safety is imperative, but it is beyond the scope of this tool kit to detail the safety procedures that need to be followed. If you have any concerns about using a ladder or climbing onto your roof, hire a professional to do the cleaning.
9. Always wear appropriate footwear and clothing.
10. Always wear fall protection.
11. Never climb onto your roof without having another person present.
12. The above cleaning solution may harm plants and trees. Cover these to protect them from the cleaning solution. Be prepared to rinse them after the roof has been cleaned.
13. Consider hiring a professional to do this work.

Regularly rinsing a metal roof with water or a diluted solution of water and dish soap (Dawn, Ajax, Palmolive) is always recommended to minimize the prevalence of organic growth on a metal roof, but organic growth may still develop. 

This blog contains two big takeaways:

• Organic growth gets its nutrients from what falls onto the roof, not from the painted metal surface.

• A diluted bleach solution can be your best solution to an ugly problem.

For more detailed information, view NCCA’s website to download Tool Kit #42, Removing Organic Growth from Prepainted Metal Roofing.